Enantiomer specific pharmacokinetics

Stereoisomers of octahydrocurcumin, the hydrogenated metabolites of curcumin, display stereoselective activity on the CYP2E1 enzyme in L-02 cells

As the final hydrogenated metabolite of curcumin, octahydrocurcumin (OHC) exhibits increased powerful bioactivities. The chiral and symmetric chemical structure indicated that there were two OHC stereoisomers, (3R,5S)-octahydrocurcumin (Meso-OHC) and (3S,5S)-octahydrocurcumin ((3S,5S)-OHC), which may induce different effects on metabolic enzymes and bioactivities. Thus, we detected OHC stereoisomers from rat metabolites (blood, liver, urine and feces) after oral administration of curcumin. In addition, OHC stereoisomers were prepared and then their different influences on cytochrome P450 enzymes (CYPs) and UDP-glucuronyltransferases (UGTs) in L-02 cells were tested to explore the potential interaction and different bioactivities. Our results proved that curcumin could be metabolised into OHC stereoisomers first. In addition, Meso-OHC and (3S,5S)-OHC exhibited slight induction or inhibition effects on CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP3A4 and UGTs. Furthermore, Meso-OHC exhibited more intensive inhibition toward CYP2E1 expression than (3S,5S)-OHC, ascribed to the different mode of binding to the enzyme protein (P < 0.05), which finally induced more effective liver protection effects in acetaminophen-induced L-02 cell injury.

Interpreting the Benefit and Risk Data in Between-Drug Comparisons: Illustration of the Challenges Using the Example of Mefenamic Acid versus Ibuprofen

Evidence-based pain therapy should rely on precisely defined and personalized criteria. This includes balancing the benefits and risks not only of single drugs but often requires complex between-drug comparisons. Non-steroidal anti-inflammatory drugs (NSAIDs) have been available for several decades and their use is described in an abundance of guidelines. Most of these guidelines recommend that ‘the selection of a particular NSAID should be based on the benefit-risk balance for each patient’. However, head-to-head studies are often lacking or of poor quality, reflecting the lower standards for clinical research and regulatory approval at the time. The inconsistency of approved indications between countries due to national applications adds to the complexity. Finally, a fading research interest once drugs become generic points to a general deficit in the post-marketing evaluation of medicines. Far from claiming completeness, this narrative review aimed to illustrate the challenges that physicians encounter when trying to balance benefits and risks in a situation of incomplete and inconsistent data on longstanding treatment concepts. Ibuprofen and mefenamic acid, the most frequently sold NSAIDs in Austria, serve as examples. The illustrated principles are, however, not specific to these drugs and are generalizable to any comparison of older drugs in daily clinical practice.

Chiral Recognition R- and RS- of New Antifungal: Complexation/Solubilization/Dissolution Thermodynamics and Permeability Assay

Novel potential antifungal of 1,2,4-triazole class have been synthesized as pure enantiomer (R-98) and racemic (RS-186). The effect of 2-hydroxypropyl-β-cyclodextrin (CD) on the solubility and permeability of RS-186 and R-98 in terms of chiral recognition was investigated. Phase solubility studies were carried out at 4 temperatures in 0-0.05 M CD concentration range for pH 2.0 and pH 7.4. AL- and AL--type phase-solubility profiles were obtained for both compounds in pH 2.0 and pH 7.4. The racemic formed more stable complexes with CD as compared to R-isomer. Disclosing of chiral discrimination was facilitated using the approach based on the complex consideration of the derived complexation/solubilization/inherent dissolution thermodynamic functions, including the differential parameters between the racemic compound and R-enantiomer. The differences in the thermodynamic parameters determined by the chirality were discussed in terms of the driving forces of the processes and the main interactions of the compounds with CD in solution. The membrane permeability of both samples in the presence of CD was accessed in order to evaluate the specificity of enantioselective transport through the lipophilic membrane. The solubility/permeability interrelation was disclosed. The investigated compounds were classified as medium permeable in pure buffers and low permeable in the presence of 0.01 M CD. The obtained results can be useful for the design of pharmaceutical products in the form of liquid formulations based on the investigated substances.

When Regional Anesthesia Met Pharmacokinetics

Pharmacokinetics of Local Anaesthetic Agents. By Tucker GT, Mather LE. Br J Anaesth 1975; 47(suppl 1):213-24 Information derived from measurements of blood concentrations of local anaesthetics can be extended by the application of pharmacokinetic analysis. A better understanding of quantitative aspects of the disposition and absorption of these drugs should assist the anaesthetist in deciding the optimal agent and dosage for regional block techniques.

Dual 2-Hydroxypropyl-β-Cyclodextrin and 5,10,15,20-Tetrakis (4-Hydroxyphenyl) Porphyrin System as a Novel Chiral-Achiral Selector Complex for Enantioseparation of Aminoalkanol Derivatives with Anticancer Activity in Capillary Electrophoresis

In this study, a complex consisting of 2-hydroxypropyl-β-cyclodextrin and 5,10,15,20-tetrakis (4-hydroxyphenyl) porphyrin, (named dual chiral-achiral selector complex) was used for the determination of two novel potential anticancer agents of (I) and (II) aminoalkanol derivatives. This work aimed at developing an effective method that can be utilized for the determination of I (S), I (R), and II (S) and II (R) enantiomers of (I) and (II) compounds through the use of a dual chiral-achiral selector complex consisting of hydroxypropyl-β-cyclodextrin and 5,10,15,20-tetrakis (4-hydroxyphenyl) porphyrin system by applying capillary electrophoresis. This combination proved to be beneficial in achieving high separation selectivity due to the combined effects of different modes of chiral discrimination. The enantiomers of (I) and (II) compounds were separated within a very short time of 3.6–7.2 min, in pH 2.5 phosphate buffer containing 2-hydroxypropyl-β-cyclodextrin and 5,10,15,20-tetrakis (4-hydroxyphenyl) porphyrin system at a concentration of 5 and 10 mM, respectively, at 25 °C and +10 kV. The detection wavelength of the detector was set at 200 nm. The LOD for I (S), I (R), II (S), and II (R) was 65.2, 65.6, 65.1, and 65.7 ng/mL, respectively. LOQ for I (S), I (R), II (S), and II (R) was 216.5, 217.8, 217.1, and 218.1 ng/mL, respectively. Recovery was 94.9–99.9%. The repeatability and reproducibility of the method based on the values of the migration time, and the area under the peak was 0.3–2.9% RSD. The stability of the method was determined at 0.1–4.9% RSD. The developed method was used in the pilot studies for determining the enantiomers I (S), I (R), II (S), and II (R) in the blood serum.

Predicting Stereoselective Disposition of Carvedilol in Adult and Pediatric Chronic Heart Failure Patients by Incorporating Pathophysiological Changes in Organ Blood Flows-A Physiologically Based Pharmacokinetic Approach

Chronic heart failure (CHF) is a systemic low perfusion syndrome resulting from impairment in the pumping function of the heart. The decrease in blood supply to body organs can potentially affect the pharmacokinetics (PK) of the drugs being administered. Carvedilol is administered as a racemic mixture and undergoes extensive stereoselective first pass metabolism. For such a drug, the pathophysiological changes occurring in CHF can have a profound impact on PK, and thus the resulting pharmacodynamic response, of both enantiomers. The aim of the current work was to predict stereoselective disposition of carvedilol after incorporating the pathophysiological changes in CHF into a whole-body physiologically based PK model using Simcyp, and to scale that model to pediatric CHF patients on a physiologic basis to investigate whether the same changes in the adult model can also be adopted for children. The developed model has successfully described PK of carvedilol enantiomers in healthy adults and in patients after the incorporation of reduced organ blood flows, as seen by the visual predictive checks and the calculated observed/predicted ratios for all PK parameters of interest. In contrast to adults, pediatric patients up to 12 years of age were better described without the reductions in organ blood flow, whereas older pediatric patients were better described after incorporating organ blood flow reductions. These findings indicate that the incorporated blood flow reductions in the adult model cannot be directly adopted in pediatrics, at least for the young ones; however, to draw definite conclusions, more data are still needed.

Antivitamins for Medicinal Applications

Antivitamins represent a broad class of compounds that counteract the essential effects of vitamins. The symptoms triggered by such antinutritional factors resemble those of vitamin deficiencies, but can be successfully reversed by treating patients with the intact vitamin. Despite being undesirable for healthy organisms, the toxicities of these compounds present considerable interest for biological and medicinal purposes. Indeed, antivitamins played fundamental roles in the development of pioneering antibiotic and antiproliferative drugs, such as prontosil and aminopterin. Their development and optimisation were made possible by the study, throughout the 20th century, of the vitamins' and antivitamins' functions in metabolic processes. However, even with this thorough knowledge, commercialised antivitamin-based drugs are still nowadays limited to antagonists of vitamins B9 and K. The antivitamin field thus still needs to be explored more intensely, in view of the outstanding therapeutic success exhibited by several antivitamin-based medicines. Here we summarise historical achievements and discuss critically recent developments, opportunities and potential limitations of the antivitamin approach, with a special focus on antivitamins K, B9 and B12 .

Trapping of muscle relaxant methocarbamol degradation product by complexation with copper(II) ion: spectroscopic and quantum chemical studies

Structural properties of methocarbamol (Mcm) were extensively studied both experimentally and theoretically using FT IR, (1)H NMR, UV-Vis., geometry optimization, Mulliken charge, and molecular electrostatic potential. Stability arises from hyper-conjugative interactions, charge delocalization and H-bonding was analyzed using natural bond orbital (NBO) analysis. Mcm was decomposed in ethanol/water mixture at 80°C to guaifenesin [(RS)-3-(2-methoxyphenoxy)propane-1,2-diol] and carbamate ion [NH2COO(-)], where the degradation mechanism was explained by trapping the carbamate ion via the complexation with copper(II) ion. The structure of the isolated complex ([Cu(NH2COO)2(H2O)]⋅4H2O) was elucidated by spectral, thermal, and magnetic tools. Electronic spectra were discussed by TD-DFT and the descriptions of frontier molecular orbitals and the relocations of the electron density were determined. Calculated g-tensor values showed best agreement with experimental values from EPR when carried out using both the B3LYP and B3PW91 functional.

Population pharmacokinetics of oral baclofen in pediatric patients with cerebral palsy

OBJECTIVE

To characterize the population pharmacokinetics (PK) of oral baclofen and assess impact of patient-specific covariates in children with cerebral palsy (CP) in order to support its clinical use.

SUBJECTS DESIGN

Children (2-17 years of age) with CP received a dose of titrated oral baclofen from 2.5 mg 3 times a day to a maximum tolerated dose of up to 20 mg 4 times a day. PK sampling followed titration of 10-12 weeks. Serial R- and S-baclofen plasma concentrations were measured for up to 16 hours in 49 subjects. Population PK modeling was performed using NONMEM 7.1 (ICON PLC; Ellicott City, Maryland).

RESULTS

R- and S-baclofen showed identical concentration-time profiles. Both baclofen enantiomers exhibited linear and dose/kg-proportional PK, and no sex differences were observed. Average baclofen terminal half-life was 4.5 hours. A 2-compartment PK model with linear elimination and transit absorption steps adequately described concentration-time profiles of both baclofen enantiomers. The mean population estimate of apparent clearance/F was 0.273 L/h/kg with 33.4% inter-individual variability (IIV), and the apparent volume of distribution (Vss/F) was 1.16 L/kg with 43.9% IIV. Delayed absorption was expressed by a mean transit time of 0.389 hours with 83.7% IIV. Body weight, a possible genetic factor, and age were determinants of apparent clearance in these children.

CONCLUSION

The PK of oral baclofen exhibited dose-proportionality and were adequately described by a 2-compartment model. Our population PK findings suggest that baclofen dosage can be based on body weight (2 mg/kg per day) and the current baclofen dose escalation strategy is appropriate in the treatment of children with CP older than 2 years of age.

Human Pharmacokinetics and Interconversion of Enantiomers of MK-0767, a Dual PPARα/γ Agonist

MK-0767, a dual peroxisome proliferator-activated receptor (PPAR) alpha/gamma agonist, has been studied as a potential treatment of type 2 diabetes and dyslipidemia. The pharmacokinetics and interconversion of (+)-(R)-MK-0767 and (-)-(S)-MK-0767 were evaluated following oral administration of each single enantiomer and the racemate to healthy subjects. The results demonstrate that, consistent with in vitro experiments, chiral inversion occurs rapidly in vivo, and interconversion equilibrium favors (+)-(R). After all treatments, a stable ratio (R/S) of 2 to 2.5 was achieved within 8 hours in most individuals, congruent with model-based estimates of interconversion half-life. In addition, the pharmacokinetics of each enantiomer were generally similar regardless of treatment. Modeling and simulation of enantiomer disposition suggest that the observed predominance of (+)-(R)-MK-0767 in plasma may result from differential volumes of distribution between (-)-(S) and (+)-(R), preferential conversion from (-)-(S) to (+)-(R), or a combination of these, but not faster clearance of (-)-(S) compared to (+)-(R).

Enantiospecific ketoprofen concentrations in plasma after oral and intramuscular administration in growing pigs

Background

Ketoprofen is a non-steroidal anti-inflammatory drug which has been widely used for domestic animals. Orally administered racemic ketoprofen has been reported to be absorbed well in pigs, and bioavailability was almost complete. The objectives of this study were to analyze R- and S-ketoprofen concentrations in plasma after oral (PO) and intra muscular (IM) routes of administration, and to assess the relative bioavailability of racemic ketoprofen for both enantiomers between those routes of administration in growing pigs.

Methods

Eleven pigs received racemic ketoprofen at dose rates of 4 mg/kg PO and 3 mg/kg IM in a randomized, crossover design with a 6-day washout period. Enantiomers were separated on a chiral column and their concentrations were determined by liquid chromatography-tandem mass spectrometry. Pharmacokinetic parameters were calculated and relative bioavailability (F_rel) was determined for S and R –ketoprofen.

Results

S-ketoprofen was the predominant enantiomer in pig plasma after administration of the racemic mixture via both routes. The mean (± SD) maximum S-ketoprofen concentration in plasma (7.42 mg/L ± 2.35 in PO and 7.32 mg/L ± 0.75 in IM) was more than twice as high as that of R-ketoprofen (2.55 mg/L ± 0.99 in PO and 3.23 mg/L ± 0.70 in IM), and the terminal half-life was three times longer for S-ketoprofen (3.40 h ± 0.91 in PO and 2.89 h ± 0.85 in IM) than R-ketoprofen (1.1 h ± 0.90 in PO and 0.75 h ± 0.48 in IM). The mean (± SD) relative bioavailability (PO compared to IM) was 83 ± 20% and 63 ± 23% for S-ketoprofen and R-ketoprofen, respectively.

Conclusions

Although some minor differences were detected in the ketoprofen enantiomer concentrations in plasma after PO and IM administration, they are probably not relevant in clinical use. Thus, the pharmacological effects of racemic ketoprofen should be comparable after intramuscular and oral routes of administration in growing pigs.

Stereoselective binding of chiral anti-diabetic drug nateglinide to plasma proteins

The binding of nateglinide (NA) enantiomers with human plasma (HP), human serum albumin (HSA) and bovine serum albumin (BSA) was investigated. The protein binding was studied over a drug concentration range of 5-100 μM at a protein concentration of 600 μM. Unbound drug concentrations were determined by direct chiral liquid chromatography using chiralcel OJ-RH column. At therapeutic drug concentrations, the protein binding of each enantiomer was >98%. The results showed that the binding of NA enantiomers was stereoselective, mutually competitive and non-linear. The binding characteristics were, however, opposite for the two most important plasma binding proteins. Opposite stereo-selectivity was observed between BSA and HSA while stereo-selectivity was identical between HSA and HP. Scatchard analysis was used to illustrate the different binding affinities of NA enantiomers to BSA, HSA and HP. The interaction between enantiomers observed in HP and serum albumins was confirmed as a competitive type interaction at the high affinity site. Scatchard analysis was used to illustrate the different binding affinities of NA enantiomers to BSA, HSA and HP.

The stereochemical configuration of flavanols influences the level and metabolism of flavanols in humans and their biological activity in vivo

Extensive epidemiological and clinical evidence associates diets high in flavanol-containing foods with cardiovascular health benefits in humans. Catechin and epicatechin, the most common flavanols in foods, are present in the diet in different enantiomeric forms. This study investigated the influence of the stereochemical configuration of flavanols on their absorption, metabolism, and biological activity. Healthy adult males were asked to consume equal amounts of the stereochemically pure flavanols (-)-epicatechin, (-)-catechin, (+)-catechin, and (+)-epicatechin (1.5mg/kg bw) in a well-defined cocoa-based, dairy-containing drink matrix, and flavanol levels were subsequently determined in plasma and 24-h urine. The results obtained show that the stereochemical configuration of flavanols has a profound influence on their uptake and metabolism in humans. In addition, we assessed the vasodilatory activity of each flavanol stereoisomer in vivo and found (-)-epicatechin to be the single stereoisomer capable of mediating a significant arterial dilation response. Importantly, this effect was independent of the classic antioxidant properties of flavanols. Overall, these results indicate that the proposed beneficial health effects associated with the consumption of flavanol-containing foods will significantly depend on the stereochemical configuration of the flavanols ingested.

Pharmacologically active compounds in the environment and their chirality

Pharmacologically active compounds including both legally used pharmaceuticals and illicit drugs are potent environmental contaminants. Extensive research has been undertaken over the recent years to understand their environmental fate and toxicity. The one very important phenomenon that has been overlooked by environmental researchers studying the fate of pharmacologically active compounds in the environment is their chirality. Chiral drugs can exist in the form of enantiomers, which have similar physicochemical properties but differ in their biological properties such as distribution, metabolism and excretion, as these processes (due to stereospecific interactions of enantiomers with biological systems) usually favour one enantiomer over the other. Additionally, due to different pharmacological activity, enantiomers of chiral drugs can differ in toxicity. Furthermore, degradation of chiral drugs during wastewater treatment and in the environment can be stereoselective and can lead to chiral products of varied toxicity. The distribution of different enantiomers of the same chiral drug in the aquatic environment and biota can also be stereoselective. Biological processes can lead to stereoselective enrichment or depletion of the enantiomeric composition of chiral drugs. As a result the very same drug might reveal different activity and toxicity and this will depend on its origin and exposure to several factors governing its fate in the environment. In this critical review a discussion of the importance of chirality of pharmacologically active compounds in the environmental context is undertaken and suggestions for directions in further research are made. Several groups of chiral drugs of major environmental relevance are discussed and their pharmacological action and disposition in the body is also outlined as it is a key factor in developing a full understanding of their environmental occurrence, fate and toxicity. This review will be of interest to environmental scientists, especially those interested in issues associated with environmental contamination with pharmacologically active compounds and chiral pollutants. As the review will outline current state of knowledge on chiral drugs, it will be of value to anyone interested in the phenomenon of chirality, chiral drugs, their stereoselective disposition in the body and environmental fate (212 references).

Stereospecific oxidation of the (S)-enantiomer of RS-8359, a selective and reversible monoamine oxidase A (MAO-A) inhibitor, by aldehyde oxidase

In a previous paper by the authors on RS-8359, a new selective and reversible monoamine oxidase A (MAO-A) inhibitor, it was reported that the (S)-enantiomer of RS-8359 is rapidly eliminated from rats, monkeys and humans as a result of the formation of a 2-oxidative metabolite. The present study investigates the properties of the enzyme responsible for the 2-oxidation of RS-8359. Subcellular localization, cofactor requirement and the inhibitory effects of typical compounds were studied using rat liver preparations. In addition, the enzyme was purified from rat liver cytosol for further characterization. The enzyme activity was localized in the cytosolic fraction without the need for any cofactor and was extensively inhibited by menadione, chlorpromazine and quinacrine. The purified enzyme was also a homodimer with a monomeric molecular weight of 140 kDa and it had an A280/A450 ratio of 5.1 in the absorption spectrum. The results suggest that the enzyme responsible for the biotransformation of RS-8359 to give the 2-keto derivative is aldehyde oxidase (EC 1.2.3.1). The reaction of aldehyde oxidase is highly stereoselective for the (S)-configuration of RS-8359 and the (9R)-configuration of cinchona alkaloids.

Chirality and pharmacokinetics: an area of neglected dimensionality?

Drug stereochemistry has, until relatively recently, been an area of neglected dimensionality with the development of the majority of synthetic chiral drugs as racemates. This situation has changed in recent years as a result of advances in the chemical technologies associated with the synthesis, analysis and preparative scale resolution of the enantiomers of chiral molecules. As a result of the application of these technologies the potential significance of the differential pharmacodynamic and pharmacokinetic properties of the enantiomers present in a racemate have become appreciated. Many of the processes involved in drug disposition, i.e. absorption, distribution, metabolism and excretion, involve a direct interaction with chiral biological macromolecules, e.g. transporters, membrane lipids and enzymes, and following administration of a racemate the individual enantiomers frequently exhibit different pharmacokinetic profiles and rarely exist in a 1:1 ratio in biological fluids. The magnitude of the differences between a pair of enantiomers observed in their pharmacokinetic parameters tends to be relatively modest in comparison to their pharmacodynamic properties. However, the observed stereoselectivity may be either amplified or attenuated depending on the organisational level, e.g. whole body, organ or macromolecular, the particular parameter represents. Differences in parameters involving a direct interaction between a drug enantiomer and a biological macromolecule, e.g. intrinsic metabolite formation clearance and fraction unbound, tend to be largest, and comparison of parameters reflecting the whole body level of organisation, e.g. half-life, clearance, volume of distribution, may well mask significant stereoselectivity at the macromolecular level. In spite of the recent interest in drug chirality relatively limited pharmacokinetic data are available for the enantiomers of a number of commonly used racemic drugs. Factors influencing the stereo-selectivity of drug disposition include: formulation and route of administration; in vivo stereochemical stability, both chemical and enzymatic; drug interactions, both enantiomeric and with a second drug; disease state; age; gender; race; and pharmacogenetics. As a result of such factors estimation of pharmacokinetic parameters, development of complex pharmacokinetic models and plasma-concentration-effect relationships based on 'total' drug concentrations following administration of a racemate are of limited value and potentially useless.

Stereoselective pharmacokinetics of ketorolac in calves after a single intravenous and oral dose

The purpose of this study was to establish the stereospecific pharmacokinetics of ketorolac (KT) in calves following a single 2 mg/kg intravenous (i.v.) and a single 8 mg/kg oral dose. Plasma concentrations were determined using a stereoselective HPLC assay. Pharmacokinetic parameters for both the stereoisomers were estimated by model-independent methods. Following an i.v. dose, the plasma concentration profiles of the stereoisomers were similar with half-lives of 5.9 +/- 5.1 h for R-KT and 6.0 +/- 4.9 h for S-KT. Clearance values for R- and S-KT after an i.v. dose were 0.0470 +/- 0.0370 and 0.0480 +/- 0.0370 L/h/kg respectively. After an oral dose, the terminal half-lives were longer than following i.v. administration with values of 14.77 +/- 3.08 and 14.55 +/- 2.95 h for R-KT and S-KT respectively. The average oral bioavailability was 86.5 +/- 20.6% for R-KT and 86.7 +/- 20.3% for S-KT. The results indicate that the stereoisomers of KT have similar pharmacokinetic profiles in calves. Although, unlike humans, bioinversion between KT stereoisomers appears minimal in calves, studies with individual isomers are needed before any firm conclusions can be drawn about this lack of KT bioinversion.

Stereoselectivity in drug metabolism

Many chiral drugs are used as their racemic mixtures in clinical practice. Two enantiomers of a chiral drug generally differ in pharmacodynamic and/or pharmacokinetic properties as a consequence of the stereoselective interaction with optically active biological macromolecules. Thus, a stereospecific assay to discriminate between enantiomers is required in order to relate plasma concentrations to pharmacological effect of a chiral drug. Stereoselective metabolism of drugs is most commonly the major contributing factor to stereoselectivity in pharmacokinetics. Metabolizing enzymes often display a preference for one enantiomer of a chiral drug over the other, resulting in enantioselectivity. The structural characteristics of enzymes dictate the enantiomeric discrimination associated with the metabolism of chiral drugs. The stereoselectivity can, therefore, be viewed as the physical property characteristic that phenotypes the enzyme. This review provides a comprehensive appraisal of stereochemical aspects of drug metabolism (i.e., enantioselective metabolism and first-pass effect, enzyme-selective inhibition or induction and drug interaction, species differences and polymorphic metabolism).

Biantennary Glycans as Well as Genetic Variants of α1-Acid Glycoprotein Control the Enantioselectivity and Binding Affinity of Oxybutynin

PURPOSE

The purpose of this study was to investigate the role of biantennary branching glycans of alpha1-acid glycoprotein (AGP) and its genetic variants in the enantioselective binding of oxybutynin (OXY).

METHOD

Human native AGP was separated using imminodiacetate-copper (II) affinity chromatography into two fractions, the A variant and a mixture of the F1 and S variants (F1-S). These fractionated AGPs were further separated by concanavalin A affinity chromatography into two fractions, with and without biantenarry glycans. An on-line high-performance liquid chromatography (HPLC) system consisting of a high-performance frontal analysis column, an extraction column, and an analytical HPLC column was developed to determine the binding affinities of OXY enantiomers for respective AGP species.

RESULTS

The total binding affinity as well as the enantiomeric selectivity of OXY in the F1-S mixed variant was significantly higher than that for the A variant, indicating that the chiral recognition ability of native AGP for the OXY enantiomers highly depends on the F1-S mixed variant. Furthermore, not only the genetic variants but also bianntenary glycans of AGP affect the binding affinity of OXY and are also responsible for the enantioselectivity.

CONCLUSIONS

Both genetic variants and glycan structures significantly contribute to the enantioselectivity and the binding affinity of OXY.

Stereochemical aspects of itraconazole metabolism in vitro and in vivo

Itraconazole (ITZ) has three chiral centers and is administered clinically as a mixture of four stereoisomers. This study evaluated stereoselectivity in ITZ metabolism. In vitro experiments were carried out using heterologously expressed CYP3A4. Only (2R,4S,2'R)-ITZ and (2R,4S,2'S)-ITZ were metabolized by CYP3A4 to hydroxy-ITZ, keto-ITZ, and N-desalkyl-ITZ. When (2S,4R,2'R)-ITZ or (2S,4R,2'S)-ITZ was incubated with CYP3A4, neither metabolites nor substrate depletion were detected. Despite these differences in metabolism, all four ITZ stereoisomers induced a type II binding spectrum with CYP3A4, characteristic of coordination of the triazole nitrogen to the heme iron (K(s) 2.2-10.6 nM). All four stereoisomers of ITZ inhibited the CYP3A4-catalyzed hydroxylation of midazolam with high affinity (IC(50) 3.7-14.8 nM). Stereochemical aspects of ITZ pharmacokinetics were evaluated in six healthy volunteers after single and multiple oral doses. In vivo, after a single dose, ITZ disposition was stereoselective, with a 3-fold difference in C(max) and a 9-fold difference in C(min) between the (2R,4S)-ITZ and the (2S,4R)-ITZ pairs of diastereomers, with the latter reaching higher concentrations. Secondary and tertiary ITZ metabolites (keto-ITZ and N-desalkyl-ITZ) detected in plasma were of the (2R,4S) stereochemistry. After multiple doses of ITZ, the difference in C(max) and C(min) decreased to 1.5- and 3.8-fold, respectively. The initial difference between the stereoisomeric pairs was most likely due to stereoselective metabolism by CYP3A4, including stereoselective first-pass metabolism as well as stereoselective elimination. However, stereoselective elimination was diminished after multiple dosing, presumably as a result of CYP3A4 autoinhibition. In conclusion, the metabolism of ITZ is highly stereoselective in vitro and in vivo.

Enantioselective 2-hydroxylation of RS-8359, a selective and reversible MAO-A inhibitor, by cytochrome P450 in mouse and rat liver microsomes

RS-8359, (+/-)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]-pyrimidine is a racemic compound with a selective and reversible monoamine oxidase A (MAO-A) inhibition activity. The substrate and product enantioselectivity with respect to 2-hydroxylation of RS-8359 enantiomers was studied using mouse and rat liver microsomes. In mice, the (S)-enantiomer was transformed to the cis-diol metabolite, whereas the (R)-enantiomer to the trans-diol metabolite. The Vmax/Km value for the formation of the cis-diol metabolite from the (S)-enantiomer was sevenfold greater than that for the formation of the trans-diol metabolite from the (R)-enantiomer. The greater Vmax/Km value for the (S)-enantiomer was due to the tenfold smaller Km value compared to that for the (R)-enantiomer. The results were in fair agreement with the previously reported low plasma concentrations of the (S)-enantiomer and the high recovery of the cis-diol metabolite derived from the (S)-enantiomer in urine after oral administration of RS-8359 to mice. Similarly to mice, in rats the (R)-enantiomer was transformed to the trans-diol metabolite, whereas the (S)-enantiomer yielded the cis-diol and trans-diol metabolites. The Vmax/Km value for the (R)-enantiomer was larger than that for the (S)-enantiomer in rats, indicating that the low plasma concentration of the (S)-enantiomer in rats might be caused by a metabolic reaction other than P450-dependent hydroxylation. CYP3A was shown to be responsible for the trans-diol formation from the (R)-enantiomer.

Stereoselective analysis of metoprolol and its metabolites in rat plasma with application to oxidative metabolism

We investigated the stereoselective kinetic disposition and metabolism of metoprolol (MET) in rats. The racemic MET (15 mg/kg) was given by oral gavage and blood samples were collected from 0 to 10h (n=6 at each time point). The enantiomeric concentrations of MET and its metabolites alpha-hydroxymetoprolol (alpha-OHM) and O-demethylmetoprolol (ODM) were determined by HPLC using a Chiralpak AD chiral column and fluorescence detection. The pharmacokinetic parameters of unchanged MET and the formation of ODM did not show to be stereoselective. In contrast, the AUC (ng h/mL) of alpha-hydroxymetoprolol isomers were higher to I'R [638.2(525.2-706.2) for 1'R2R and 659.6(580.4-698.1) for 1'R,2S, mean, (95%CI)] than to I'S products [58.3(47.4-66.1) for 1'S,2R and 57.1(44.7-67.9) for 1'S,2S, mean, (95%CI)]. We conclude that the kinetic disposition of unchanged MET and the formation of ODM are not enantioselective in rats but the metabolism of alpha-OHM yields predominantly the 1'R-product.

Stereoselective pharmacokinetics of tetrahydropalmatine after oral administration of (?)-enantiomer and the racemate

Tetrahydropalmatine (THP) is a biologically active ingredient isolated from a traditional Chinese herb Rhizoma corydalis (yanhusuo). THP is a racemic mixture which contains 50% of the (+) and 50% of (-) enantiomer. The (-) enantiomer accounts for most of the analgesic effects. Plasma concentrations of THP enantiomers were analyzed by chiral high-performance liquid chromatography (HPLC) on a Chiralcel OJ column with quantification by UV at 230 nm. The method was used to determine the pharmacokinetics of THP enantiomers in rats and dogs after oral administration of rac-THP or (-)-THP. The pharmacokinetic profiles of the two enantiomers after dosing with rac-THP were significantly different both in rats and dogs. The mean C(max) and AUC(0-infinity) values in rats were 1.93 +/- 0.36 microg/ml and 6.65 +/- 2.34 microg x h/ml for the (-) enantiomer, and 1.11 +/- 0.25 microg/ml and 2.03 +/- 0.45 microg x h/ml for the (+) enantiomer. The mean C(max) and AUC(0-infinity) in dogs were 1.60 +/- 0.81 microg/ml and 9.88 +/- 2.58 microg x h/ml for the (-) enantiomer, while 0.36 +/- 0.21 microg/ml and 1.22 +/- 0.40 microg x h/ml for the (+) enantiomer. rac-THP at 40 mg/kg and (-)-THP at 20 mg/kg had very similar plasma concentration-time profiles, and C(max), AUC(0-infinity), and t(1/2) of the (-) enantiomer in both rats and dogs, indicating that the two treatments were equivalent with respect to the pharmacokinetic properties of the (-) enantiomer.

Stereoselective pharmacokinetics of RS-8359, a selective and reversible MAO-A inhibitor, by species-dependent drug-metabolizing enzymes

RS-8359, (+/-)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]pyrimidine selectively and reversibly inhibits monoamine oxidase A (MAO-A). After oral administration of rac-RS-8359 to rats, mice, dogs, monkeys, and humans, plasma concentrations of the (R)-enantiomer were greatly higher than were those of the (S)-enantiomer in all species studied. The AUC((R)) to AUC((S)) ratios were 2.6 in rats, 3.8 in mice, 31 in dogs, and 238 in monkeys, and the (S)-enantiomer was almost negligible in human plasma. After intravenous administration of RS-8359 enantiomers to rats, the pharmacokinetic parameters showed that the (S)-enantiomer had a 2.7-fold greater total clearance (CL(t)) and a 70% shorter half-life (t(1/2)) than those for the (R)-enantiomer but had no difference in distribution volume (V(d)). No significant difference in the intestinal absorption rate was observed. The principal metabolites were the 2-keto form, possibly produced by aldehyde oxidase, the cis-diol form, and the 2-keto-cis-diol form produced by cytochrome P450 in rats, the cis-diol form in mice, RS-8359 glucuronide in dogs, and the 2-keto form in monkeys and humans. Thus, the rapid disappearance of the (S)-enantiomer from the plasma was thought to be due to the rapid metabolism of the (S)-enantiomer by different drug-metabolizing enzymes, depending on species.

Enantioselective Kinetic Disposition of Fenoprofen in Rats with Experimental Diabetes or Adjuvant-Induced Arthritis

This study was aimed to investigate the influence of diabetes or arthritis on the enantioselective metabolism and kinetic disposition of fenoprofen in rats with streptozotocin-induced diabetes or Mycobacteriumtuberculosis adjuvant-induced arthritis. Animals received i.v. 10 mg/kg racemic fenoprofen and blood samples were collected up to 24 h thereafter, with 5 animals studied at each time point. Plasma concentrations of the fenoprofen enantiomers were determined by HPLC. Diabetic and arthritic animals showed significant differences when compared with respective controls for the following pharmacokinetic variables of the (+)-(S)-fenoprofen eutomer: area under the plasma concentration time curve, total clearance and volume of distribution. The results indicate that experimental diabetes and adjuvant-induced arthritis influence the fenoprofen enantioselective metabolism.

Analysis of competitive binding of ligands to human serum albumin using NMR relaxation measurements

The competitive binding of two ligands, ibuprofen (IBP) and salicylic acid (SAL), to human serum albumin (HSA) was studied by using nuclear magnetic resonance (NMR) relaxation measurements. When the concentration of one ligand was increased in the solution containing IBP, SAL and HSA, the fractions of free IBP and SAL were increased because of the competitive binding. The 1H relaxation rates (R1) of both ligands were subsequently decreased. If a ligand is in fast exchanging between the free and bound forms, the observed 1H relaxation rate is a weighted average of that for the free ligand and the protein-ligand complex. The concentrations of the free and bound ligands can be quantitatively derived from the relaxation rates. The results presented in this work revealed that IBP and SAL shared certain low-affinity binding sites on the HSA molecule, in addition to the same high-affinity binding site of AIII.

Influence of rheumatoid arthritis in the enantioselective disposition of fenoprofen

To investigate the influence of rheumatoid arthritis on the stereoselective disposition of fenoprofen administered as a racemic mixture, eight patients with rheumatoid arthritis receiving calcium rac-fenoprofen (200 mg/8 h) and 7 healthy volunteers given single oral dose (600 mg) were investigated. Serial blood samples and urine were collected from zero to 24 h after fenoprofen (FEN) administration. The following differences were observed between the (+)-(S) and (-)-(R)-FEN in the patients with rheumatoid arthritis (means 95% CI, Wilcoxon test, P < 0.05): C(max) 14.1 (12.5-15.8) versus 3.6 (2.5-4.7) microg/ml; AUC(ss) (0-8) 80.5 (67.3-93.7) versus 12.1 (8.8-15.4) microg.h/ml; Cl(T)/f 1.3 (1.0-1.5) versus 9.1 (6.5-11.8) l/h; and t(1/2) 3.1 (2.3-3.9) versus 1.2 (0.8-1.6) h. The Cl(T)/f of (-)-(R)-FEN was reduced in patients with rheumatoid arthritis when compared to healthy volunteers: 9.1 (6.5-11.8) versus 17.4 (13.9-20.9) l/h; P < 0.05 Mann-Whitney test. The administration of rac-FEN as a single dose to healthy volunteers or multiple doses to patients with rheumatoid arthritis resulted in lower Cl(T)/f for the (+)-(S)-FEN. The lower Cl(T)/f of (-)-(R)-FEN observed for patients with rheumatoid arthritis is consistent with lower clearance by inversion, although other metabolic pathways, drug interactions, and bioavailability of the individual enantiomers may also contribute to the difference.

Role of biological matrices during the analysis of chiral drugs by liquid chromatography

The review article covers advances of chiral drugs analysis by high-performance liquid chromatography (HPLC) methods achieved during last 10 years. Emphasis is given to various aspects of influence of biological matrix in pharmacodynamics, pharmacokinetics, HPLC analysis. Discussed is composition of main biological matrices from the point of view of potential interferences to above-mentioned fields of study. Beside typical analytical approaches to chiral recognition in HPLC, sample pretreatment and/or clean-up by conventional extraction procedures, column switching (CSW) techniques using restricted access materials (RAMs), microdialysis (MCD) is discussed. Measurement of unbound drug concentration and discussion of column maintenance and remedy is an additional source of information and field where knowledge on complex properties and interactions of biological matrix is usefully applicable.

Stereoselective pharmacokinetics and metabolism of flobufen in guinea pigs

Stereoselective aspects of pharmacokinetics and metabolism of a chiral nonsteroidal antiinflammatory drug, flobufen, 4-(2', 4'-difluorobiphenyl-4-yl)-2-methyl-4-oxobutanoic acid, were studied in male guinea pigs after p.o. administration of racemic flobufen (rac-flobufen) at a dose of 10 mg/kg. Blood samples were collected at intervals over 16 h after the administration of rac-flobufen for the quantification of flobufen enantiomers and their respective metabolites in plasma by chiral high-performance liquid chromatography (HPLC). Compartmental pharmacokinetic analysis was used to determine pharmacokinetic parameters of R- and S-flobufen. The plasma concentrations of the S- and R-enantiomers differed significantly during the experimental period. The S/R-enantiomeric ratio in 7plasma reached a maximum value of 10.1 at 240 min postdose. The oral clearance value of R-flobufen was five times higher than S-flobufen. The other pharmacokinetic parameters (K(e), T(1/2), V(SS)/F, MRT) of the enantiomers also differed substantially. All four stereoisomers of the dihydrometabolite of flobufen were detected in plasma with varying concentrations. Metabolite 17203 [4-(2,4-difluorophenyl)-phenylacetic acid] exhibited a relatively longer residence time compared to that noted for the enantiomers of the parent compound. Pharmacokinetics of the flobufen enantiomers were stereoselective in guinea pigs. The metabolism of flobufen was complex. However, metabolite 17203 seemed to be the main metabolite of flobufen that may be responsible for its relatively long-lasting antiphlogistic and immunomodulatory effects.

Drug Binding Analysis of Human α1-Acid Glycoprotein Using Capillary Electrophoresis

Drug-plasma protein binding analysis is indispensable for drug development and clinical use. However, conventional methods for binding analyses were not suitable for small amounts of proteins because of large sample requirements. On the other hand, high-performance frontal analysis/capillary electrophoresis (HPFA/CE) consumes very small sample volumes, and is useful for ligand-binding study of small amounts of proteins. In this study, HPFA/CE was used in a drug-binding study of alpha 1-acid glycoprotein (AGP) subtypes in which plasma concentrations change dynamically to elucidate the effects of structural variation on drug binding. Binding study on desialyrated AGP revealed that (S)-enantiomer selectivity in propranolol-AGP binding was caused by sialic acid residues, while neither sialic acid nor galactose caused the enantioselectivity of verapamil binding to AGP. Biantennary glycans slightly suppressed disopyramide binding to AGP, whereas the glycans did not have any influence on propranolol and verapamil binding. Disopyramide and verapamil were selectively bound to the A variant rather than the F1S variant. The A variant showed larger enantioselective binding to disopyramide, but not to verapamil.

Racemization and degradation of thioridazine and thioridazine 2-sulfone in human plasma and aqueous solutions

We present two methods for the enantioselective analysis of thioridazine (THD) and thioridazine 2-sulfone (THD 2-SO(2)) in human plasma based on liquid-liquid extraction with diethyl ether and chiral resolution of the enantiomers on Chiralpak AD and Chiralcel OD-H columns, respectively. After validation, the methods were used to study the degradation and racemization of both drug and metabolite. Our results showed that both enantiomers of THD and THD 2-SO(2) were stable at varying temperatures, pH, and ionic strengths; however, solubility problems for THD and THD 2-SO(2) enantiomers were observed, mainly at pH 8.5. The influence of light on the stability of the THD and THD 2-SO(2) enantiomers was also studied. Degradation of the THD enantiomers was observed under UV light (254 and 366 nm) while THD 2-SO(2) enantiomers were stable at these wavelengths and also when exposed to visible light.

Specific determination of unbound oxacillin in protein solution with cefoperazone by high-performance frontal analysis with chemiluminescence detection

Unbound oxacillin concentrations in human serum albumin (HSA) solutions in the presence or absence of cefoperazone were determined using high-performance frontal analysis coupled with chemiluminescence detection (HPFA-CL). The HPFA was performed on an ISRP column with 67 mM potassium phosphate buffer of pH 7.4 and ionic strength of 0.17 as the mobile phase. The luminol-H(2)O(2)-Co(2+) system was employed in the chemiluminescence detection. The detection was highly specific for oxacillin in the presence of cefoperazone. Although both drugs in HSA solutions co-eluted in the same region in HPFA, cefoperazone did not interfere with the determination of unbound concentration of oxacillin. In the solution of 100 microM HSA and 11.33 micro M oxacillin the bound percentage of oxacillin to HSA was estimated as 80.5%. Addition of 30.98 micro M cefoperazone into the HSA-equilibrated solution produced little effect on the protein binding of oxacillin. In the presence of 154.9 micro M cefoperazone, however, the bound percentage of oxacillin was significantly reduced. This specific method could be applied to the investigation of drug-drug interaction in protein binding.

The importance of stereoselective determination of drugs in the clinical laboratory

About 56% of the drugs currently in use are chiral compounds, and 88% of these chiral synthetic drugs are used therapeutically as racemates. Only a few of these drugs qualify for a stereospecific determination in a clinical laboratory for therapeutic drug monitoring of patients. If the qualitative and quantitative pharmacokinetic and pharmacodynamic effects are similar, the enantiomers do not need to be separated. However, if the metabolism of the different stereoisomers is handled by different enzymes which are either polymorphic or can be induced or inhibited, and if their pharmacodynamic effects have differences either in strength or in quality, enantiospecific analysis is urgently needed. Unfortunately, there are many racemic drugs where the stereospecificity of the metabolism and/or the pharmacodynamic effects of the enantiomers is not known today. For these drugs, there is a great need for studies concentrating on these differences to improve treatment of the patients.

Impact of stereoselectivity on the pharmacokinetics and pharmacodynamics of antiarrhythmic drugs

Many antiarrhythmic drugs introduced into the market during the past three decades have a chiral centre in their structure and are marketed as racemates. Most of these agents, including disopyramide, encainide, flecainide, mexiletine, propafenone and tocainide, belong to class I antiarrhythmics, whereas verapamil is a class IV antiarrhythmic agent. Except for encainide and flecainide, there is substantial stereoselectivity in one or more of the pharmacological actions of chiral antiarrhythmics, with the activity of enantiomers differing by as much as 100-fold or more for some of these drugs. The absorption of chiral antiarrhythmics appears to be nonstereoselective. However, their distribution, metabolism and renal excretion usually favour one enantiomer versus the other. In terms of distribution, plasma protein binding is stereoselective for most of these drugs, resulting in up to two-fold differences between the enantiomers in their unbound fractions in plasma and volume of distribution. For disopyramide, stereoselective plasma protein binding is further complicated by nonlinearity in the binding at therapeutic concentrations. Hepatic metabolism plays a significant role in the elimination of these antiarrhythmics, accounting for >90% of the elimination of mexiletine, propafenone and verapamil. Additionally, in most cases, significant stereoselectivity is observed in different pathways of metabolism of these drugs. For some drugs, such as propafenone and verapamil, the stereoselectivity in metabolism is further complicated by nonlinearity in one or more of the metabolic pathways. Further, the metabolism of a number of chiral antiarrhythmics, such as mexiletine, propafenone, encainide and flecainide, cosegregates with debrisoquine/sparteine hydroxylation phenotype. Therefore, it is not surprising that a wide interindividual variability exists in the metabolism of these drugs. Excretion of the unchanged enantiomers in urine is an important pathway for the elimination of disopyramide, flecainide and tocainide. The renal clearances of both disopyramide and flecainide exceed the filtration rate for these drugs, suggesting the involvement of active tubular secretion. However, the stereoselectivity in the renal clearance of these drugs, if any, is minimal. Similarly, there is no stereoselectivity in the renal clearance of tocainide, a drug that undergoes tubular reabsorption in addition to glomerular filtration. Overall, substantial stereoselectivity has been observed in both the pharmacokinetics and pharmacodynamics of chiral antiarrhythmic agents. Because the effects of these drugs are related to their plasma concentrations, this information is of special clinical relevance.

Effect of omeprazole on oral and intravenous RS-methadone pharmacokinetics and pharmacodynamics in the rat

The effect of omeprazole on oral and intravenous (iv) RS-methadone pharmacokinetics and pharmacodynamics was studied in awake, freely moving rats, which were divided in four groups: oral RS-methadone (3 mg/kg) was given (a) to a control group (CO(oral)) (n = 65) and (b) to an omeprazole pretreated group (OP(oral)) (n = 77), and iv RS-methadone (0.35 mg/kg) was administered (c) to a control group (CO(iv)) (n = 86) and (d) to an omeprazole pretreated group (OP(iv)) (n = 86). Omeprazole (2 mg/kg) was given iv 2 h before RS-methadone. Plasma concentrations of RS-methadone (Cp) were determined by high-performance liquid chromatography and analgesic response by tail flick for 0-180 min (oral) and 0-120 min (iv). RS-Methadone rate of absorption (mean +/- SE) was faster in OP(oral) (k(01) = 0.31 +/- 0.04 min(-1)) than in CO(oral) (k(01) = 0.05 +/- 0.007 min(-1)), consequently plasma peak concentrations (C(max)) were greater (197.41 +/- 33.70 ng/mL versus 83.54 +/- 7.97 ng/mL) and the time to reach C(max) (t(max)) was shorter (11.23 +/- 1.32 min versus 39.18 +/- 1.74 min). Mean area under the Cp-time curve (AUC(0-infinity)) and hence bioavailability of oral RS-methadone were increased by omeprazole without significant changes in the elimination. Omeprazole did not affect the pharmacokinetics of iv RS-methadone. The changes of the analgesic effect of RS-methadone as a function of time were similar in all four groups. In the CO(oral) group, Cp and analgesic effect were defined by the E(max) model. The relationship between Cp and drug effect in the OP(oral) group showed a counterclockwise hysteresis (k(e0) of 0.018 +/- 0.006 min(-1)). For the iv groups (CO(iv) and OP(iv)), the Cp-analgesic effect relationship was described by an E(max) sigmoid model and omeprazole did not affect the pharmacodynamic parameters. It is concluded that omeprazole causes an increase in the bioavailability of oral RS-methadone without modifying the analgesic response but affecting the Cp-effect relationship.

Influence of input rate on the stereospecific and nonstereospecific first pass metabolism and pharmacokinetics of metoprolol extended release formulations

The purpose of this study was to examine the influence of input rate on the stereoselective and nonstereoselective pharmacokinetics of metoprolol, alpha-hydroxymetoprolol, and its acid metabolite. Extended release formulations (100 mg) of metoprolol with varying release rates (e.g., slow (S), moderate (M), and fast (F)) and an oral solution (OS, 50 mg) were administered to normal, healthy extensive metabolizers. Serial blood samples were collected over 48 h, plasma was obtained, and subsequently analyzed by a validated HPLC method with fluorescence detection. The mean T(max) of metoprolol after the S, M, F (4.43, 4.00, 3.14 h, respectively) was found to be different ( P < 0.05) as compared to the OS (2.07 h). The ratio of alpha-hydroxymetoprolol/metoprolol was higher for the OS (1.26) vs. the S, M, and F (1.02, 0.96, 0.99). The S/R enantiomer ratios of the concentration for metoprolol, ACMB, and alpha-hydroxmetoprolol were calculated at each time point and showed a significant difference ( P < 0.05) in the absorption phase (1-4 h) vs. terminal phase (8-16 h) for fast input (solution and fast extended release formulations). Based on these results, it would appear that input rate influences the pharmacokinetics of metoprolol, its metabolites, and their enantiomers.

The development of single-isomer molecules: why and how

Until relatively recently the three-dimensional nature of drug molecules has been largely neglected, with approximately 25% of marketed drugs being mixtures of agents rather than single chemical entities. These mixtures are not combinations of drugs but mixtures of stereoisomers, generally racemates of synthetic chiral drugs. The individual enantiomers present in such mixtures frequently differ in both their pharmacodynamic and pharmacokinetic profiles as a result of stereochemical discrimination on interaction with chiral biological macromolecules (enzymes and receptors). The use of such mixtures may present problems if their adverse effects are associated with the less active stereoisomer or do not show stereoselectivity. In addition, interactions between enantiomers may occur such that the observed activity of the racemate is not simply the product of the effects of the individual enantiomers. Since the mid-1980s there has been an ongoing "racemate-versus-enantiomer" debate with the potential advantages of single-isomer products, including improved selectivity of action and potential increase in therapeutic index, being highlighted. As a result, regulatory authorities have issued guidelines for dealing with chiral molecules, and the number of single enantiomer agents presented for evaluation has increased. Racemic mixtures may still be developed but require justification such that the risk-benefit ratio may be assessed. In addition to new chemical entities, a number of "old" mixtures are being re-examined as potential single-isomer products, the chiral switches, with the potential for an improved therapeutic profile and possibly new indications. However, for the majority of agents currently marketed as mixtures, relatively little is known concerning the pharmacological or toxicological properties of the individual enantiomers.

Binding study of desethyloxybutynin using high-performance frontal analysis method

Plasma protein binding of N-desethyloxybytynin (DEOXY), a major active metabolite of oxybutynin (OXY), was investigated quantitatively and enantioselectively using high-performance frontal analysis (HPFA). An on-line HPLC system which consists of HPFA column, extraction column and analytical column was developed to determine the unbound concentrations of DEOXY enantiomers in human plasma, in human serum albumin (HSA) solutions, and in human alpha1-acid glycoprotein (AGP) solutions. DEOXY is bound in human plasma strongly and enantioselectively. The unbound drug fraction in human plasma samples containing 5 microM (R)- or (S)-DEOXY was 1.19 +/- 0.001 and 2.33 +/- 0.044%, respectively. AGP plays the dominant role in this strong and enantioselective plasma protein binding of DEOXY. The total binding affinity (nK) of (R)-DEOXY and (S)-DEOXY to AGP was 2.97 x 10(7) and 1.31 x 10(7) M(-1), respectively, while the nK values of (R)-DEOXY and (S)-DEOXY to HSA were 7.77 x 10(3) and 8.44 x 10(3) M(-1), respectively. While the nK value of (S)-DEOXY is weaker than that of (S)-OXY (1.53 x 10(7) M(-1)), the nK value of (R)-DEOXY is 4.33 times stronger than that of (R)-OXY (6.86 x I0(6) M(-1)). This suggests that the elimination of an ethyl group weakens the binding affinity of the (S)-isomer because of the decrease in hydrophobicity, while the binding affinity of the (R)-isomer is enhanced by the decrease in steric hindrance. The total binding affinity of DEOXY to HSA is much lower than that of DEOXY-AGP binding as well as OXY-HSA binding (2.64 x 10(4) and 2.19 x 10(4) M(-1) for (R)-OXY and (S)-OXY, respectively). The study on competitive binding between OXY and DEOXY indicated that DEOXY enantiomers and OXY enantiomers are all bound competitively at the same binding site of AGP molecule.